UK : In the dog days of summer, it can seem hard to sleep. Additionally, as stifling heatwaves become more frequent, the nights can be oppressively hot with no a refreshing breeze to ease the agony. At least you may believe what your senses tell you: nights are indeed growing warmer.
At or just after dawn, weather stations often take the minimum temperature reading for the day. Records at a few locations in the UK go as far back as 150 years. Scientists have discovered that nighttime temperatures have increased significantly since the Victorian era, even after taking into account little changes in tools and techniques over time. In the majority of the investigated records, nighttime temperatures are actually increasing more quickly than midday temperatures. Why is that so?
The UK has experienced fewer really cold nights in recent milder winters. The coldest winter days are typically not as cold as the coldest winter nights. The average lowest nighttime temperature has risen disproportionately more quickly than the average maximum daytime temperature as a result of their removal.
Due to climate change, the UK experiences hotter weather more frequently during the summer. In the UK, extreme daytime and nighttime temperatures have increased by roughly 2°C over the past 150 years during heatwaves.
Heat islands in cities
However, even a brief hot spell makes it possible for warm evenings to last even after daytime temperatures have returned to more normal levels, especially in cities. This results in more hot nights than hot days overall. Because concrete and asphalt collect and release heat from the sun more slowly at night than do outlying rural areas, city people experience much greater nighttime temperatures. The urban heat island effect is what is causing this.
Even though the evidence is conflicting, it has been suggested that condensation trails left by aeroplanes may have increased nighttime temperatures by limiting the amount of heat that can escape the atmosphere’s surface layers to space.
A lot may be learned about how nighttime temperatures have evolved from data from two of the UK’s oldest weather stations, the Durham University Observatory and the Radcliffe Observatory in Oxford, both of which have records dating back to 1841.
At Oxford, the warmest night of the year averaged 16.6°C between 1911 and 1920. Over the past ten years, the average temperature increased by more than 2°C to 18.8°C. Despite two hot summers in the 1970s, warm nights, defined as those where temperatures remain over 15°C, presently average 20 per year in Oxford, more than twice the average at that time (1975 and the notorious 1976). There are approximately twice as many warm nights in Central London as there are in Oxford.
At Oxford, only twelve nights have been above 20°C since 1814, as of this writing (so-called tropical nights). The majority of those—50%—have happened in the previous 25 years, with the most extreme of all occurring in July 2016 at 21.2°C. This might even be soon surpassed. Although Oxford’s metropolitan area has expanded since 1814, the location of the weather station hasn’t changed much since the 1830s, and the rise in mean temperature caused by the urban heat island effect over the duration of record keeping is probably only about 0.2°C.
Heatwaves in the north and north-eastern parts of England are shorter and less powerful than those in the east and south-east, and hot nights are consequently less common. In north-east England, evenings hotter than 15°C have been far less frequent over the past ten years, averaging only six or seven per year, or one-third of Oxford’s frequency, according to data from the Durham University Observatory. But even here, there are now four times as many warm evenings as there were in the 1970s. The warmest night of the year in Durham has increased by 2°C in the last 10 years, from an average of 14.6°C a century ago to 16.9°C, very comparable to Oxford.
However, the hottest night (18.4°C), which was registered there on July 12 2022, was just half a degree Celsius behind the all-time high: 18.9°C, also achieved in July 2016. Fortunately, nights exceeding 20°C are as of yet unknown in the extensive Durham record. Also maybe soon to be surpassed is that. Additionally, on July 12, 2022, Sheffield experienced its warmest nighttime minimum temperature in the 140 years of record keeping, coming in at 20.5°C.
Even in the Republic of Ireland, which is renowned for having an equable climate, a hot period in July 2021 produced the first tropical night in 20 years, with a minimum temperature at Valentia Observatory in the extreme south-west of Kerry that lingered at 20.5°C. Only six instances of such warm nights are known to have occurred in Ireland.
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Particularly in the south and east of England, heatwaves are becoming increasingly frequent and powerful. By 2100, temperatures exceeding 40°C, more than a degree above the current UK national record (38.7°C, recorded in Cambridge in July 2019), are likely to happen occasionally, according to a Met Office research.
Nighttime temperatures will gradually rise while daytime extremes continue to rise. At Brighton, East Sussex, during the August 1990 heatwave, 23.9°C now holds the record for the UK’s highest lowest temperature (hottest night). There are a few additional places where 23°C has also been noted overnight, including central London.
Without very significant cuts in fossil fuel consumption, nighttime temperatures in some regions during warm weather won’t drop below 25°C by the turn of the century, and potentially even much earlier. Currently, a hot day is one with a daytime temperature of 25°C. The summers of the future will be long, hot, and sleepless unless we reduce carbon emissions.